1. Field
The following description relates to an orthogonal frequency division multiplexing (OFDM) receiver for receiving an OFDM-based signal and, more particularly, to an OFDM receiver and method for compensating for phase and gain imbalance between in-phase (I) and quadrature (Q) channels.
2. Description of the Related Art
OFDM is a frequency-division multiplexing scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme at a low symbol rate. One key principle of OFDM is that since low symbol rate modulation schemes suffer less from intersymbol interference caused by multipath propagation, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, which allows high spectral efficiency.
More specifically, an OFDM transmitter divides a single high-rate data stream into a number of low-rate parallel data streams, modulates each sub-carrier carrying each parallel data stream using a conventional modulation scheme, such as quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM), performs Inverse Fast Fourier Transform (IFFT) to convert the modulated sub-carrier into a time-domain signal, and inserts a pilot symbol, a training symbol or a cyclic prefix (CP) to generate a complex-valued signal. In this case, assuming that real and imaginary parts of the complex-valued signal are respectively referred to as an I-channel and a Q-channel, carriers with a phase difference of 90 degrees are used to carry the I-channel and Q-channel.
On the other hand, an OFDM receiver receives the carriers from the OFDM transmitter, produces a baseband complex-valued signal by multiplying an earlier-received one of the carriers in reverse order in the OFDM transmitter with a local oscillator (LO) signal with a phase difference of 90 degrees from the carrier, performs a frequency difference compensation process and a time synchronization process, performs FFT to be converted into a frequency-domain signal, performs an equalization process, and extracts an information signal.
In this case, however, if the phase difference between signals applied to the I-channel and Q-channel in the transmitter or receiver is not equal to 90 degrees or the signals do not have the same amplitude, I/Q imbalance occurs, resulting in severe degradation in the performance of the OFDM system.